The present invention relates to non-thermal plasma reactors and more particularly relates to structural conductor elements for non-thermal plasma reactors.
Certain compounds in the exhaust stream of a combustion process, such as the exhaust stream from an internal combustion engine, are undesirable in that they must be controlled in order to meet government emissions regulations, Among the regulated compounds are hydrocarbons, soot particulates, and nitrogen oxide compounds (NOx). There are a wide variety of combustion processes producing these emissions, for instance, coal-or oil-fired furnaces, reciprocating internal combustion engines (including gasoline spark ignition and diesel engines), gas turbine engines, and so on. In each of these combustion processes, control measures to prevent or diminish atmospheric emissions of these emissions are needed.
Industry has devoted considerable effort to reducing regulated emissions from the exhaust streams of combustion processes. In particular, it is now usual in the automotive industry to place a catalytic converter in the exhaust system of gasoline spark ignition engines to remove undesirable emissions from the exhaust by chemical treatment. Typically, a xe2x80x9cthree-wayxe2x80x9d catalyst system of platinum, palladium, and rhodium metals dispersed on an oxide support is used to oxidize carbon monoxide and hydrocarbons to Water and carbon dioxide and to reduce nitrogen oxides to nitrogen. The catalyst system is applied to a ceramic substrate such as beads, pellets, or a monolith. When used, beads are usually porous, ceramic spheres having the catalyst metals impregnated in an outer shell. The beads or pellets are of a suitable size and number in the catalytic converter in order to place an aggregate surface area in contact with the exhaust stream that is sufficient to treat the compounds of interest. When a monolith is used, it is usually a cordierite honeycomb monolith and may be pre-coated with gamma-alumina and other specialty oxide materials to provide a durable, high surface area support phase for catalyst deposition. The honeycomb shade, used with the parallel channels running in the direction of the flow of the exhaust stream, both increases the surface area exposed to the exhaust stream and allows the exhaust stream to pass through the catalytic converter without creating undue back pressure that would interfere with operation of the engine.
When a spark ignition engine is operating under stoichiometric conditions or nearly stoichiometric conditions (referred to as non-oxidizing conditions or as producing a non-oxidizing atmosphere) with respect to the fuel-air ratio (just enough oxygen to completely combust the fuel, or perhaps up to 0.3% excess oxygen), a xe2x80x9cthree-wayxe2x80x9d catalyst has proven satisfactory for reducing emissions. Unburned fuel (hydrocarbons) and oxygen are consumed in the catalytic converter, and the relatively small amount of excess oxygen does not interfere with the intended operation of the conventional catalyst system.
However, it is desirable to operate the engine at times under lean burn conditions, with excess air (referred to as oxidizing conditions or as producing an oxidizing atmosphere), in order to improve fuel economy. Under lean burn conditions, conventional catalytic devices are not very effective for treating the NOx in the resulting oxygen-rich exhaust stream.
The exhaust stream from a diesel engine also has a substantial oxygen content, from perhaps about 2-18% oxygen, and, in addition, contains a significant amount of particulate emissions. The particulate emissions, or soot, are thought to be primarily carbonaceous particles. It is also believed that other combustion processes result in emissions that are difficult or expensive to control because of, for instance. dilute concentrations of the compounds to be removed from the effluent stream or poor conversion of the compounds using conventional means.
In spite of efforts over the last decade to develop a effective means for reducing NOx to nitrogen under oxidizing conditions in a spark ignition. gasoline engine or in an diesel engine, the need for improved conversion effectiveness has remained unsatisfied. Moreover, there is a continuing need for improved effectiveness in treating emissions from any combustion process, particularly for treating the soot particulate emissions from diesel engines.
An alternative way to treat the hydrocarbon, particulate, or NOx emissions in an exhaust or effluent stream. would be to destroy such emissions using a non-thermal plasma. Plasma is regarded as the fourth state of matter (ionized state of matter) Unlike thermal plasmas, non-thermal plasmas (NTPs) are in gaseous media at near-ambient temperature and pressure but have electron mean energies considerably higher than other gaseous species in the ambient environment. NTP species include electrically neutral gas molecules, charged particles in the form of positive ions, negative ions, free radicals and electrons, and quanta of electromagnetic radiation (photons). These NTP species are highly reactive and can convert hazardous gases to non-hazardous or less hazardous and easily managed compounds through various chemical reaction mechanisms. In contrast to, thermal processes (such as thermal plasma), an NIP process directs electrical energy to induce favorable gas chemical reactions, rather than using the energy to heat the gas. Therefore, NTP is much more energy-efficient than thermal plasma.
NTPs can be generated by electric discharge in the gas or injection of electrons into the gas by an electron beam. Electron beams must be accelerated under a high vacuum and then transferred through special windows to the reaction site. The reaction site must be sized with respect to the penetration depth of the electrons. It is much more difficult to scale-up the size of an electron beam reactor than an electric discharge reactor. Therefore, electron beam reactors are less favored than electric discharge reactors.
Among the various types of electric discharge reactors, pulse corona and dielectric barrier (silent) discharge reactors are very popular for their effectiveness and efficiency. However, pulse corona reactors have the major disadvantage of requiring special pulsed power supplies to initiate and terminate the pulsed corona. Consequently, dielectric barrier discharge has become a fast growing technology for pollution control.
Cylindrical and planar reactors are to common configurations for dielectric barrier discharge reactors. Both of these configurations are characterized by the presence of one or more insulating layers in a current path between two metal electrodes, in addition to the discharge space. Other dielectric barrier discharge reactors include packed-bed discharge reactors, glow discharge reactors, and surface discharge reactors.
Current dielectric barrier discharge NTP reactor designs are based upon the use of one or more structural dielectric ceramic pieces coated with a conductive material arranged to form. dielectric barrier-conductor-dielectric barrier configurations. Reactor assembly generally comprises providing electrical. connections to the prepare element, which is typically wrapped with a mat, stuffed into a cylindrical housing or a housing half shell and secured thereto. A stacked shape non-thermal plasma reactor and element for use with diesel engines and the like is disclosed in commonly assigned U.S. patent application Ser. No. 09/511,590 (Attorney Docket No. DP-300505) entitled xe2x80x9cDesign and Method of Manufacturing A Plasma Reactor For Treating Auto Emissionsxe2x80x94Stacked Shapesxe2x80x9d, which is hereby incorporated by reference herein in its entirety. Disclosed therein is a non-thermal plasma reactor prepared from a formed shape of dielectric. material used as a building block for creating the region of the non-thermal plasma reactor wherein plasma is generated. The formed shape defines an internal cell in the plasma reactor having an exhaust passage for flowing exhaust gas to be treated therethrough. A conductive print forming an electrode and connector is disposed on at least one wall of each of the cells. and outer insulative plates, disposed on opposite ends of the multi-cell stack, are provided to protect the conductive print.
Problematically, structural ceramic substrates provide relatively poor dimensional control with respect to thickness and camber. For example, dimensional thickness and camber of ceramic substrates may vary, such as from +/xe2x88x9210% and +/xe2x88x920.4%, respectively, resulting in variations in dielectric barrier thickness and gaps. This dimensional variation limits the practical operating range for the non-thermal plasma reactor in applications such as after-treatment of diesel exhaust emissions.
Commonly assigned U.S. patent application Ser. No. 09/741,764 now U.S. Pat. No. 6,537,507 (Attorney Docket No. DP-302475) entitled xe2x80x9cNon-Thermal Plasma Reactor Design And Method-Single Structural Dielectric Barrierxe2x80x9d, which is hereby incorporated by reference herein in its entirety, discloses a single structural dielectric barrier non-thermal plasma reactor element prepared with structural ceramics. The reactor includes an element having at least one cell comprising a conductor forming an electrode and connector, an exhaust passage for flowing gas there through, and a single structural dielectric barrier, so that during reactor operation, a non-thermal plasma is formed in the exhaust passages for treating fluid as it passes through the exhaust passages. Individual cells of the element comprise a conductor-single structural dielectric barrier-exhaust passage-conductor arrangement.
Structural ceramics comprise a significant portion of the cost factor for current NTP reactor designs based on structural ceramics. In addition, ceramic materials typically used for such applications, including cordierite, mullite, and alumina, have mid-level dielectric constants in the range of about 5.3 to about 10, limiting the ability to reduce the overall size of the NTP reactor
What is needed in the art is an improved NTP reactor and method that can be manufactured at reduced cost while meeting application performance and durability requirements.
The present non-thermal plasma reactor has a structural conductor element comprising a base conductor support; and a high dielectric constant (xe2x80x9chigh kxe2x80x9d) barrier layer supported by and substantially surrounding the base conductor support to form a structural conductor non-thermal plasma reactor element. The dielectric barrier layer may comprise a double dielectric barrier or a single dielectric barrier. Embodiments comprising a single dielectric barrier are advantageous for providing low, cost fabrication, high, power per area reactor performance, and compact construction. However, embodiments comprising a double dielectric barrier are considered to have slightly higher constituent conversion efficiency than for the single dielectric barrier embodiments.
The structural conductor element may comprise a variety of shapes including plates or sheets, half-box shapes (C-shapes), or comb shapes combined to provide inter-digitized tine elements, among others.
The dielectric barrier layer comprises a material applied to the base conductor support, such as by coating or laminating. In an alternate embodiment, the dielectric barrier layer is formed by converting the base conductor support via electrochemical. thermal or chemical methods to form a dielectric barrier layer from outer surfaces of the base conductor support.
The present invention provides non-thermal plasma reactors comprising a multi-cell stack of structural base conductor elements, including, but not limited to, structural base conductor plates, C-shaped structural base conductors, and inter-digitized tine shaped elements. The inter-digitized tines have rounded corners and are prepared from comb-shaped structural base conductors.
The present structural conductor NTP reactors may be used in a variety of applications wherein NOx reduction is desired, including, but not limited to, industrial NOx reduction abatement applications and transportation applications such as mobile diesel applications.
Advantageously, the high k dielectric barrier layer on the structural conductor base increases the capacitance of the NTP reactor, with respect to prior NTP reactor designs using structural ceramics as the conductor support, due to reduced dielectric barrier thickness and higher k level.
Advantageously, the present structural conductor NTP reactors provide improved dimensional control and lower dielectric material and manufacturing costs.
These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings.